Transmitters for mobile radios normally comprise a baseband unit having an analog interface for outputting a complex baseband signal with an inphase component I and a quadrature component Q. This interface is standardized and is called the I/Q interface of the baseband unit. The baseband unit processes the information or data to be sent, which is/are in the form of a bit sequence. The bits are coded on the basis of a selected modulation type and result in a complex baseband signal which thus represents a sequence of data symbols.
The analog I/Q interface of the baseband unit then has a radio-frequency unit connected to it which converts the analog baseband signal to the transmission frequency. In known embodiments, the conversion is performed by a special mixer called an I/Q modulator.
In more recent transmission arrangements, however, the baseband signal is converted to the RF transmission frequency using a “digital modulator” whose inputs are designed to supply digital information or data preferably in the form of digital baseband signals, for example digital I and Q signals. Such digital modulators are used primarily in transmission devices in the GSM mobile radio standard. The modulation type used by the standard is frequency shift keying, which is particularly simple to implement in digital modulators. However, it is not possible to connect a digital modulator to a known analog standard I/Q interface on a baseband unit on account of the analog output signals from the baseband unit in the case of these transmission devices.